Carbonaceous electrodes for lithium cells

ABSTRACT

A rechargeble battery of alkali metal, organic electrolyte type has a low surface area particulate carbonaceous electrode intercalable with the alkali metal and experiences little capacity loss upon intercalation of the carbonaceous electrode with the alkali metal. The carbonaeous electrode may include a multi-phase composition including both highly graphitized and less graphitized phases or may include a single phase, highly graphitized composition which has been subjected to interlation of lithium at above about 50° C. Incorporation of an electrically conductive filamentary material such as carbon black intimately interspersed with the carbonaceous composition minimizing capacity loss upon repeated cycling.

BACKGROUND OF THE INVENTION

The present invention relates to non-aqueous lithium cells, such asstorage batteries.

Non-aqueous lithium cell storage batteries typically include an anode ofmetallic lithium, a lithium electrolyte prepared from a lithium saltdissolved in one or more organic solvents and a cathode of anelectrochemically active material, typically a chalcogenide of atransition metal. During discharge, lithium ions from the anode passthrough the liquid electrolyte to the electrochemically active materialof the cathode where the ions are taken up with the simultaneous releaseof electrical energy. During charging, however, the flow of ions isreversed so that lithium ions pass from the electrochemically activematerial through the electrolyte and are plated back onto the lithiumanode.

During each discharge/charge cycle small amounts of lithium andelectrolyte are consumed by chemical reactions at newly createdsurfaces. As lithium inherently tends to form high surface area peaks ordendrites as it is plated back onto the anode, this reactive conditionis aggravated. Furthermore, the dendritic peaks continue to grow untilthey eventually contact the cathode which causes the cell to fail.Additional amounts of lithium do not cohesively plate onto the anodeduring the charge cycle and result in the formation of spongy depositsnear the anode surface. As these deposits are not in electricallyconductive contact with the anode, they eventually detract from thecapacity of the cell.

One approach to minimizing the consumption of lithium is to prevent thegrowth of lithium dendrites and spongy deposits so that only a lowsurface area layer is deposited. One method of accomplishing this is toprovide a sheet-like porous separator on the lithium surface and applysubstantial pressure on the separator, and hence on the anode.Typically, this pressure is applied as an inter-electrode pressure, alsoreferred to as "stack pressure". This approach minimizes the dendriticand spongy growths of the lithium, and helps to insure that a lowsurface area plating is deposited. However, only cells with cylindricalsymmetry can be made to withstand this large pressure with a thin metalcasing. Rectangular and coin-shaped cells would require very thickcasings in order to withstand this pressure without excessive flexing,thereby resulting in a larger battery and increased cost.

Only very expensive separators are available which are porous yetprevent dendritic penetration by lithium, and which are able towithstand the very large cell pressures which are developed. Even withthese separators, however, there is a risk that the separator will bepunctured by dendritic growth, so that only long recharge times may beused. Also, low discharge rates increase the chances of dendriticseparator puncture during charging, thereby limiting the number ofcharge/discharge cycles which may be obtained.

Even when porous separators and stack pressure are used, a smallpercentage of lithium is still consumed during each discharge/chargecycle. Thus, in order to attain a practical cell lift, it is necessaryto include a substantial excess of lithium in the cells, therebysignificantly increasing their cost and size.

Moreover, lithium metal is extremely reactive and has a low meltingpoint. With lithium cells of large size there is a danger that the heatgenerated during abnormal cell operation may lead to melting of thelithium anode. Such melting would not only render the cell inoperative,but could also lead to direct contact between the molten lithium and thecathode material, resulting in a vigorous reaction that could rupturethe cell casing.

In addition, the use of lithium metal as the anode material usuallyrequires that a toxic salt, LiAsF₆, be used in the electrolyte in orderto obtain optimum cell performance. The LiAsF₆ apparently contributes toformation of coatings on the lithium which enhances the performances ofthe cell. The use of this toxic substance, however, presents danger bothduring manufacture and in those situations where the cell casing mayrupture.

Thus, there exists a need for a rechargeable cell which will provide theadvantages provided by cells having lithium metal anodes, but which willnot have the drawbacks associated with these types of cells. Oneapproach has been to replace the lithium metal anode with a carbon anodesuch as coke or graphite intercalated with lithium metal to form Li_(x)C. In operation of the cell, lithium passes from the carbon through theelectrolyte to the cathode where it is taken up just as in a cell with ametallic lithium anode. During recharge, the lithium is transferred backto the anode where it reintercalates into the carbon. Because nometallic lithium is present in the cell melting of the anode cannotoccur even under abuse conditions. Also, because lithium isreincorporated in the anode by intercalation rather than by plating,dendritic and spongy lithium growth cannot occur.

This technique, however, has encountered numerous problems. As Li_(x) Cis a reactive material which is difficult to handle in air, it ispreferably produced in-situ in a cell. In doing so, however, lithium andcell electrolyte are consumed in an irreversible process. Thisirreversible process results in an initial capacity loss for the cellwhich reduces the cell's overall performance. Another problem with thisapproach is that the cell exhibits a progressive loss of capacity overnumerous charge/discharge cycles. This progressive loss is commonlyreferred to as "capacity fade".

Accordingly, there are still needs for further improvements in cellshaving carbon electrodes.

SUMMARY OF THE INVENTION

The present invention addresses these needs.

One aspect of the present invention provides a rechargeable batterycomprising an alkali metal such as lithium, a first electrodeintercalable with an alkali metal, a counterelectrode capable ofreversibly incorporating the alkali metal and an electrolyte includingan organic solvent and a salt of the alkali metal. The alkali metal inthe cell typically is incorporated in the first electrode, thecounterelectrode or both. The first electrode preferably includes acomposition including carbon, desirably in particulate form, thecomposition having a surface area subsequent to intercalation with thealkali metal which is substantially similar to the surface area of thecomposition prior to intercalation with the alkali metal. Desirably, atleast a portion of the composition is carbon having a degree ofgraphitization greater than about 0.40.

The term "degree of graphitization" refers to a parameter of themicrostructure further defined below, having a numerical value between 0and 1.0. In general, carbon having a high degree of graphitization has amore ordered microstructure more closely resembling the microstructureof graphite, whereas carbon having a low degree of graphitization has aless ordered microstructure more closely resembling that of coke. Carbonhaving a high degree of graphitization provides significant advantageswith respect to charge capacity or maximum value of x in Li_(x) C₆, andalso with respect to voltage stability during operation. However,attempts to use a carbon having a degree of graphitization above about0.40 as an active material in an electrode of an alkali metal celltypically result in substantial irreversible reactions which causesubstantial initial capacity losses. Although the present invention isnot limited by any theory of operation, it is believed that thehigh-graphitization carbon can undergo a substantial increase in surfacearea during initial intercalation of the alkali metal, and that the highinitial capacity loss associated with highly graphitized carbon iscaused at least in part by this increase in surface area. Regardless ofthe mechanism of operation, intercalable carbon electrodes having asurface area subsequent to intercalation with the alkali metal which issubstantially similar to the surface area prior to intercalation havebeen associated with little initial capacity loss in these rechargeablebatteries.

Preferably, the alkali metal comprises lithium. The particulatecomposition of the first electrode may comprise a first phase and asecond phase intimately admixed with the first phase. Desirably,substantially every particle of the composition includes both the firstand second phases. The first phase desirably has a relatively highdegree of graphitization, preferably above about 0.40, more preferablyabove about 0.80 and most preferably about 1.0. The second phase of thecomposition may comprise a carbonaceous material having a relatively lowdegree of graphitization, desirably less than about 0.40. Electrodesincorporating the preferred two-phase compositions are substantiallyresistant to increase in surface area during alkali metal intercalationand provide low initial capacity loss.

Alternatively, the composition may consist essentially of carbon havinga degree of graphitization greater than about 0.40 which has beenlithiated at a temperature greater than about 50° C. Initial lithiationof highly graphitized carbon electrodes at these elevated temperaturesminimizes the initial capacity loss. Preferably, the lithiationtemperature is between about 55° C. and about 70° C.

A further aspect of the present invention provides a rechargeablebattery having a first electrode and a counterelectrode each capable ofreversibly incorporating an alkali metal such as lithium and anelectrolyte comprising an organic solvent and a salt of the alkali metalwherein the first electrode comprises a particulate compositionincluding carbon, and an electrically conductive filamentary materialinterspersed with the carbon-containing composition. The addition ofelectrically conductive filamentary materials to a particulateintercalable carbon electrode substantially suppresses capacity fadeupon repeated charge/discharge cycling of the battery. It has also beenfound that incorporation of electrically conductive filamentarymaterials into intercalable carbon electrodes substantially reduces theneed to maintain pressure on the electrode assemblies. Thus, where thepreferred conductive filamentary materials are present in thecarbonaceous electrode, the cell can operate satisfactorily withoutphysical pressure on the anode and without appreciable capacity fade.This represents a major advance in that any attempt to operate withoutpressure on the anode would normally lead to rapid and severe capacityfade. Carbonaceous compositions such as graphite and coke have highelectrical conductivity in their own right. Indeed, these materials havebeen used heretofore as conductivity enhancers in electrodes formed fromother particulate materials. It is accordingly surprising that additionof another conductive material to carbonaceous electrodes would offerany performance benefit. Desirably, the filamentary material includescarbon black, preferably having a surface area of less than about 50 m²/g. In preferred batteries in accordance with this embodiment of thepresent invention, the first electrode includes between about 1 wt. %and about 12 wt. % of the filamentary material, between about 4 wt. %and about 7 wt. % of the filamentary material being more preferred.

Yet another aspect of the present invention provides a rechargeablebattery having a first electrode intercalable with an alkali metal suchas lithium, a counterelectrode intercalable with the alkali metal and anelectrolyte comprising an organic solvent and a salt of the alkali metalwherein the first electrode comprises a particulate compositionincluding carbon having a surface area of less than about 10 m² /g andmost preferably less than about 8 m² /g. Desirably, the carbon has asurface area of less than about 6 m² /g, a surface area of less thanabout 4 m² /g being more preferable, and less than about 2 m² /g beingmost preferred. This aspect of the present invention incorporates thediscovery that the surface area of alkali metal intercalablecarbonaceous material included in the electrode has a substantial effecton the initial capacity loss incurred by the cell. This is so even withcarbonaceous materials which do not incur an increase in surface areaupon intercalation of the alkali metal, such as carbonaceous materialswith low graphitization.

Another aspect of the present invention provides a process of making arechargeable battery comprising the steps of intercalating an alkalimetal such as lithium into a carbon-containing composition includingcarbon having a degree of graphitization in above about 0.40, at least aportion of said intercalating step being performed at a temperatureabove about 50° C., forming said composition into a first electrode andassembling said first electrode with an electrochemically-activecounterelectrode and an electrolyte including a salt of said alkalimetal in a cell housing. Preferably, the intercalation step is conductedafter the electrode-forming and assembling steps, and the alkali metalis intercalated into said composition while said composition is in saidelectrode in said cell housing.

These and other objects will become apparent, as will a betterunderstanding of the structure and operation of the present invention,when reference is made to the description which follows taken with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic drawing of a portion of a rechargeable batteryin accordance with the present invention during an intermediate stage inthe manufacturing process.

FIG. 2 is a further view of the cell of FIG. 1 during a further stage ofthe process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A process according to one embodiment of the present invention includesthe step of providing a first electrode or anode formed from aparticulate composition including carbon together with a filamentary,electrically conductive material such as carbon black. As used in thisdisclosure, the term "carbon" should be understood as referring to formsof carbon other than diamond. Different forms of carbon which are atleast partially crystalline can be characterized by their respectivedegrees of graphitization. As used in this disclosure, the term "degreeof graphitization" refers to the value g according to the formula:##EQU1## where d (002) is the spacing between the graphitic layers ofthe carbon in the crystal structure measured in Angstrom units. Thespacing d between graphite layers is measured by standard X-raydiffraction techniques. The positions of diffraction peaks correspondingto the (002), (004) and (006) Miller indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. This technique may be employedeven where the sample includes intimately admixed plural phases havingdiffering degrees of graphitization. The term "mean degree ofgraphitization" as used herein with reference to a two-phase material,refers to a value for degree of graphitization calculated from thespacing d(002) determined by subjecting the two-phase material to thesame X-ray diffraction technique without attempting to separate thephases from one another.

In the process according to this embodiment, the particulatecarbonaceous composition has a first phase of carbon with a relativelyhigh degree of graphitization, typically about 1.00, and a second phasewith a relatively low degree of graphitization, typically less thanabout 0.40. The mean degree of graphitization desirably is above about0.40. Both the first and second phases are included in substantiallyevery particle. One such two-phase carbonaceous material is generallyknown as isotropic graphite. Isotropic graphite can be made by mixingfinely milled graphite, having a degree of graphitization of about 1.0and a particle size of about 1 micron, with a petroleum pitch binder,and then heating the mixture so as to convert the pitch to coke or apartially graphitized form of carbon. One suitable form of isotropicgraphite is available under the designations EC-110 from Graphite Sales,Inc. of Chagrin Falls, Ohio, U.S.A. Another form of carbonaceousmaterial which can be used is known as spherical graphite. Sphericalgraphite may be made by injecting droplets of fluidized coke into afurnace at a temperature above the graphitization temperature of carbon.The resulting material has substantially a graphitic phase which mayinclude small graphitic domains or grains at different orientations, anda less graphitized phase which may be present as an interstitial phase,at the grain boundaries. One suitable spherical graphite is availablefrom Superior Graphite Co. of Chicago, Ill., U.S.A. under thedesignation 9400 series spherical graphite.

Desirably, the surface area of the particulate composition, whenassembled into the cell, is less than about 15 m² /g, and more desirablyless than about 10 m² /g as determined by the Brunauer-Emmett-Teller or"BET" method. Carbons having surface areas of less than about 8 m² /gare preferred, less than about 6 m² /g more preferred, and less thanabout 4 m² /g most preferred. Desirably, however, the particulatecarbonaceous composition has a surface area of at least about 0.03 m²/g.

Desirably, the anode contains between about 1 wt. % and about 12 wt. %of the filamentary, electrically conductive material; additions ofbetween about 4 wt. % and about 7 wt. % of said filamentary material aremore desirable. Desirably, the filamentary material is provided in theform of carbon black. Preferred carbon blacks are filamentary in thatthey consist of numerous particles connected together. When viewed undermagnification, each set of connected particles is similar in appearanceto a string of pearls. More preferred are filamentary carbon blackswhich are less than about 50 m² /gm; filamentary carbon blacks havingsurface areas of about 40 m² /gm are most preferable. Carbon blackshaving these characteristics include acetylene black and battery black.

The particulate carbonaceous composition and the filamentary, conductivematerial are admixed with one another and formed into an anode of thedesired shape, so that within the anode the filamentary material 80 isinterspersed with the particles 70 of the carbonaceous composition, asschematically indicated in FIG. 1. A sheet-like anode 20 may befabricated on a substantially inert but electrically conductive anodecurrent collector 85 by a process as disclosed in co-pending, commonlyassigned U.S. Patent Application No. 07/204,072 filed June 8, 1988. Thedisclosure of said '072 application is hereby incorporated by referenceherein. This method includes the steps of dispersing anelectrochemically active material and a binder in a solvent to form aslurry. The slurry is then deposited in a layer on a sheet-like currentcollector substrate. The substrate for a carbonaceous electrode mayconsist of a metallic foil or expanded metal such as stainless steel,nickel or copper. Once the solvent in the layer has substantiallyevaporated, the layer may be densified by compacting on the substrate toprovide an electrode. In fabrication of the anode according to thismethod, the particulate carbonaceous composition and the filamentarymaterial may be incorporated in a common slurry, and the slurry may beagitated to mix these components intimately before the slurry isdeposited and dried.

In order to form a rechargeable cell, the anode is combined with otherelements to form a layered or "sandwich" structure 100 as generallyshown in FIG. 1. Thus anode 20, formed from a carbon-containingcomposition by a method such as described above, is assembled with anelectrochemically active cathode or counterelectrode 40 capable ofreversibly incorporating lithium. Counterelectrode 40 may incorporate aparticulate material which is lithium-intercalable or otherwise capableof reversibly reacting with lithium. The particulate material isdisposed on a sheet-like electrically conductive cathode currentcollector 45. The particulate, lithium intercalable cathode material maybe a transition metal chalcogenide. Preferred compositions includemolybdenum sulfides, vanadium oxides and manganese oxides. MoS₂, V₆ O₁₃,Mo₆ S₈ and MnO₂ are more preferred, with MnO₂ being most preferred. Theparticular forms of manganese dioxide disclosed in co-pending, commonlyassigned U.S. Patent Application No. 07/217,668 filed July 11, 1988 areespecially preferred. The disclosure of said '668 application is herebyincorporated by reference herein. The particulate cathode material maybe distributed on the sheet-like current collector 45 by a slurrying andcoating process similar to that discussed above in connection with theanode. A sheet-like porous, electrically insulating separator 50, whichmay be a microporous polypropylene or a polymeric mesh, is interposedbetween cathode 40 and anode 20.

A sheet of lithium metal 30 is placed between cathode 40 and separator50 so that the lithium sheet lies adjacent to, and in contact with, theanode 20. Preferably, the size of the lithium metal sheet 30 is chosenso that the surface of the sheet is co-extensive with the surface of theanode 20, and the thickness of the sheet 30 is chosen so that thecorrect amount of lithium is present for the intercalation reaction asdiscussed below.

Sandwich structure 100 may further include an additional separator 55disposed on cathode current collector 45. This sandwich structure may bewound around a metallic center post 110, thus forming the sandwichstructure into a convoluted, spiral configuration. In this spiralconfiguration, the additional separator 55 lies between the anodecurrent collector 85 of the sandwich structure on one turn of the spiraland the cathode current collector 45 on the next adjacent turn, thusmaintaining the anode 20 and cathode 40 electrically insulated from oneanother. The sandwich structure may be wound onto the center post 110under some tension, where tension facilitates the winding process. Thistension may produce a compressive load between neighboring turns of thespiral structure. Compressive load between components in a cell is alsoreferred to as "stack pressure" However, there is no need to apply anyparticular degree of stack pressure, and the winding tension may beentirely omitted.

The spiral assembly is then fitted into a cell casing 10 and the cellcasing 10 is closed by cell cap 120. The anode and cathode currentcollector are electrically connected by conventional means (not shown)with cell casing 10 and cap 120, respectively. Casing 10 and cap 120 areinsulated from each other and serve as terminals for the finished cell.

An electrolyte, preferably including a lithium salt or otherlithium-containing compound dispersed in a suitable organic solvent andcompatible with both the cathode and anode materials, is added to thecell, typically through an opening 130 in the cap which is subsequentlysealed. Desirably, the electrolyte solvent is capable of wetting theseparators and particulate materials. The electrolyte solvent preferablyincludes ester solvents, such as propylene carbonate (PC), ethylenecarbonate (EC), or mixtures thereof. When the solvent comprises both PCand EC, the ratio of PC to EC, by volume is preferably about 1:3 toabout 3:1, more preferably about 1:2 to 2:1, and even more preferably,about 1:1. Other solvents may be used such as 2-methyl tetrahydrofuran(2-MTHF), tetrahydrofuran, sulfolane, dimethylsulfite, monoglyme(1,2-dimethoxyethane), diglyme, triglyme, tetraglyme, p-dioxane,1,3-dioxane, dimethoxymethane, diethylether and trimethoxyethane. Of thelower viscosity solvents, 2-MTHF is preferred. One useful electrolytesolvent includes about 75% 2-MTHF, 12.5% PC and 12.5% EC, all by volume.References in this disclosure to percentages of solvent ingredients byvolume should be understood as referring to the volume of the individualingredients prior to mixing. Suitable electrolyte salts include LiAsF₆ ;LiPF₆ ; LiC10₄ ; LiBF₄ ; LiB(C₆ H₅)₄ ; LiCF₃ F; LiAlCl₄ ; LiBr; andmixtures thereof. The less toxic salts are more preferred.

The addition of the electrolyte to the cell causes the lithium metal insheet 30 to intercalate into the carbonaceous composition of anode 20,as the lithium metal has a higher electrochemical potential than theanode. In effect, the lithium sheet 30 and anode 20 constitute atemporary cell having a lithium electrode and a carbonaceous electrode.Because sheet 30 is electrically connected to anode 20 by the physicalcontact between these elements, this temporary cell is short-circuited.Accordingly, the temporary cell discharges, with lithium passing fromthe high-potential electrode (sheet 30) to the lower potential electrode(anode 20). In this embodiment of the invention, this initial lithiationprocess desirably is conducted at about room temperature (20° C.) orbelow. This initial lithiation process continues until the lithium metalin sheet 30 is totally consumed by the carbon of anode 20 in theformation of Li_(x) C₆, or until the anode 20 has become saturated withlithium, whichever occurs first. Desirably, the amount of lithium insheet 30 is equal to the amount of lithium required to saturate anode20, so that the lithium sheet is entirely consumed and anode 20 is fullysaturated with lithium. Typical isotropic graphite compositions willtake up between 0.5 and 1 mole of lithium for each 6 moles of carbon inthe composition, i.e., isotropic graphite typically will intercalate Liup to a value of x between 0.5 and 1.0 in the formula Li_(x) C₆. Thecarbon black incorporated in anode 20 also will take up some lithium,typically up to about x=0.5 in the formula Li_(x) C₆ , i.e., 1 mole ofLi for each 12 moles of carbon black.

The preferred particulate carbon-containing compositions used to formthe anode in this embodiment does not exfoliate to any appreciabledegree during the first intercalation of lithium into the composition.As used in this application the term "exfoliate" refers to a change inthe material resulting in an increase in its surface area subsequent tointercalation with lithium metal as compared to its surface area priorto intercalation. Thus, the surface area of the particulate carbonaceouscomposition subsequent to intercalation with lithium metal issubstantially equal to the surface area which existed prior tointercalation with lithium metal. The tendency of a carbon-containingmaterial to exfoliate is largely dependent upon the degree ofgraphitization of the carbon material. Carbon which is highly graphitichas an organized layered structure wherein the layers are easilyseparated or exfoliated.

The substantial absence of exfoliation observed with the preferredcarbonaceous compositions is thus surprising because these compositionsincorporate a highly graphitized phase, which phase would be expected toundergo substantial exfoliation. Although the present invention is notlimited by any theory of operation, it is believed that thesubstantially nongraphitic phase surrounding the highly graphitizedphase in each particle of the preferred isotropic graphite materialsacts to reinforce the highly graphitized phase in some way and thus toprevent exfoliation of the highly graphitized phase.

Cells according to this aspect of the present invention, incorporatingthe preferred particulate carbonaceous compositions such as isotropic orspherical graphite in a form where the particulate composition has arelatively low surface area as discussed above undergo relatively littleelectrolyte decomposition during the initial lithiation. It is believedthat the electrolyte decomposition reaction is a reaction of lithiumwith the electrolyte solvent at the surface of the carbonaceousparticles. It is further believed that this reaction results information of a passivating film on the surface of the carbon particleand that this passivating film prevents further reaction. In any event,in the absence of exfoliation, the amount of electrolyte decompositionis substantially proportional to the surface area of the carbonparticles in the anode. Thus, it is believed that the relatively lowdegree of electrolyte decomposition observed with the preferredparticulate carbonaceous compositions results both from the low surfacearea of the particulate composition before the initial intercalationprocess and from the substantial absence of new surface area created byexfoliation during the initial lithiation.

The carbon black employed as the conductive filamentary material has arelatively high surface area per unit mass. However, the amounts ofcarbon black utilized in preferred embodiments of the present inventionare relatively small, and hence electrolyte solvent reactions and lossof capacity caused by reaction at the carbon black surface ordinarilyare insignificant.

Once the lithium sheet 30 has been substantially consumed and theparticulate, carbonaceous composition of the anode has been saturated bylithium, the cell is in a charged condition and ready for use. The cellis employed in the normal fashion. During discharge, anode 20 iselectrically connected to cathode 40 via an external electrical loadconnected to cell casing 10 and cell cap 120 and hence connected acrossthe anode current collector 85 and cathode current collector 45. Duringdischarge, lithium passes from the particulate carbonaceous compositionof anode 20 through the electrolyte and to the electrochemically activematerial of cathode 40, where the lithium is intercalated into thecathode material. During recharge, the process is reversed under theinfluence of an externally applied potential so that lithium iswithdrawn from the cathode material and re-intercalated into theparticulate carbonaceous composition of the anode 20.

Cells incorporating the preferred isotropic graphite or sphericalgraphite particulate compositions have relatively stable voltage duringdischarge. The total cell voltage at any given state of charge ordischarge is the arithmetic difference between the potential of thecathode material relative to a fixed reference material such as metalliclithium and the voltage or electrochemical potential of the partiallylithiated carbon composition relative to the same reference. Theelectrochemical potential of isotropic graphite relative to a lithiumreference varies by less than about 0.2 volts over the major portion ofthe cell capacity from x=0.2 to the maximum attainable value of x inLi_(x) C₆. Thus, during discharge from the fully charged state (maximumattainable value of x) to a substantially discharged state (xapproximately equal to 0.2) the variation in the total cell voltage willbe approximately equal to the variation inherent in the cathodematerial.

Moreover, cells according to this embodiment of the invention areresistant to capacity fade or progressive loss of capacity over repeatedcharge and discharge cycles. Typically, the capacity fade amounts toabout 0.1% or less of the cell capacity for each charge/discharge cycle.Provided that the conductive filamentary material such as carbon blackis incorporated in the anode, acceptable resistance to capacity fade isprovided even where there is no pressure on the anode. This isparticularly surprising inasmuch as cells incorporating particulatecarbonaceous materials ordinarily undergo substantial capacity fadeunless pressure on the anode is maintained during cycling, as bymaintaining inter-electrode pressure or "stack pressure".

During the initial lithiation process, any stack pressure which may havebeen induced by the winding process is substantially relieved, as thelithium sheet disappears. However, because it is not necessary tomaintain stack pressure in order to achieve acceptable cyclingcharacteristics, this does not present a problem. Stated another way,the ability of cells according to preferred embodiments of theinvention, incorporating the filamentary conductive material to operatewithout stack pressure greatly facilitates use of a consumable orsacrificial mass of lithium such as sheet 30 as a vehicle forintroducing lithium into the cell during assembly. There is accordinglyno need to lithiate either the anode material or the cathode materialbefore assembly of the cell. Lithiated carbonaceous anode materialstypically are unstable and difficult to handle. Moreover, many of themost useful cathode materials such as the manganese oxides are alsounstable in air when fully lithiated. The ability to use a consumablemass or sheet is of particular advantage in production of cellsincorporating cathode materials which, when lithiated, are not stable inair.

A cell according to a further embodiment of the invention issubstantially the same as that described above, except that the cellincorporates a particulate, carbonaceous composition which consistsessentially of highly graphitized carbon, i.e., carbon having a degreeof graphitization above about 0.40 and desirably above about 0.80. Mostpreferably, cells according to this embodiment of the inventionincorporate a particulate, carbonaceous composition having a degree ofgraphitization approximately equal to 1.00, i.e., a carbonaceouscomposition consisting essentially of graphite. Typically, each particleof such a composition includes only one phase. Desirably, theparticulate, carbonaceous composition, as initially incorporated intothe cell, has relatively low surface area, within the same ranges asdiscussed above in connection with the two-phase composition. Also, theanode in this embodiment desirably incorporates the same filamentaryconductive material such as carbon black as discussed above, and insubstantially the same proportions.

The manufacturing process used to make a cell in accordance with thisembodiment may include the same sequence of steps as discussed above.However, in a process employing the mono-phase, highly graphitizedparticles, the initial litiation process should occur at a temperatureabove about 50° C., preferably above about 55° C. and most preferablybetween about 55° and 70° C. Desirably, the entire initial lithiationstep is conducted at temperatures within these preferred ranges. Thus,the cell, and preferably the electrolyte, are heated to a temperaturewithin the preferred ranges before the electrolyte is introduced intothe cell, and maintained at this elevated temperature during the initiallithiation process until some or all of the lithium in the consumablemass or sheet has been intercalated into the carbonaceous composition ofthe anode. When the graphitic carbonaceous composition is initiallyintercalated at such an elevated temperature, the electrolyte solventdecomposition reaction and the consequent consumption of lithium aresubstantially reduced in comparison with that which would occur duringlithiation at room temperature (about 21° C.) or below. This result isparticularly surprising inasmuch as elevated temperatures would beexpected to accelerate the decomposition reaction. Moreover, subsequentdelithiation and relithiation of the carbon composition do not result insubstantial further decomposition reactions and capacity loss. Thus, thetemperature prevailing during the first lithiation of the material has aprofound effect on the capacity loss caused by the decompositionreaction where the particulate, carbonaceous composition is asubstantially mono-phase, highly graphitized carbon. Likewise,exfoliation during the initial lithiation is substantially suppressed byconducting the lithiation within the preferred temperature ranges. Oncethe initial lithiation has been so conducted, there is no substantialfurther exfoliation during subsequent lithiations on repeated charge anddischarge cycles.

Although the present invention is not limited by any theory ofoperation, it is believed that the elevated temperature results in ahigher degree of decomposition reaction per unit surface area of thecarbonaceous composition than would occur at room temperature. It isfurther believed, however, that the substantial absence of exfoliationmaintains the initial, relatively low surface area of the carbonaceouscomposition and thus substantially limits the decomposition reaction andloss of capacity. Thus, the net effect of high-temperature lithiation isa reduction in the overall reaction. It is further believed that theexfoliation observed during initial lithiation at relatively lowtemperatures with mono-phase, highly graphitic carbons is caused atleast in part by intracrystalline stresses arising from introduction oflithium. The highly graphitized carbon when at an elevated temperature,may have an expanded crystal lattice and therefore suffer less stress orelse may be able to withstand stresses more readily without exfoliation.

Cells according to this embodiment of the invention provide advantagesgenerally similar to those discussed above. In particular, highlygraphitic, monophase carbon compositions such as pure graphite can becharged and discharged over the range of x=0 to about x=1 in Li_(x) C₆and hence provide particularly good capacity per unit mass. Thecarbonaceous material exhibits little change in electrochemicalpotential relative to a standard reference such as lithium over therange from about x=0.2 to x=1. Typically, the variation inelectrochemical potential of the carbonaceous material throughout thisrange is less than about 0.2 volts. Moreover, where the conductivefilamentary material is included, the cells cycle acceptably withoutstack pressure.

A cell according to a further embodiment of the invention mayincorporate as the particulate, carbonaceous composition a substantiallynon-graphitic carbon, i.e., a carbon having a degree of graphitizationless than about 0.40. Suitable substantially non-graphitic carbonsinclude coke and particularly petroleum coke. In other respects, thecells may be substantially similar to those discussed above. Thus, theanode desirably incorporates the conductive filamentary material inadmixture with the particulate, carbonaceous composition. To minimizethe decomposition reaction and initial loss of capacity, the surfacearea of the particulate composition desirably is relatively low, andpreferably within the preferred ranges discussed above. Themanufacturing process used to make cells according to this embodiment ofthe invention may be substantially as discussed above. However, becausesubstantially non-graphitic carbon is substantially resistant toexfoliation during initial lithiation at substantially any practicallithiation temperature, the lithiation desirably is conducted at aboutroom temperature or below. Although cells according to this embodiment,incorporating the substantially non-graphitic carbon provide goodresistance to exfoliation and initial capacity loss, these cells aregenerally less preferred from the standpoint of specific capacity andvoltage variation. Thus, substantially non-graphitic carbon ordinarilycan take up lithium only to the extent of about x=0.5 in the formulaLi_(x) C₆ an ordinarily exhibits substantial variation in itselectrochemical potential relative to a fixed reference over its entirecharge and discharge range.

Numerous variations and combinations of the features discussed above canbe employed. For example, the sacrificial or consumable mass of lithiummay be omitted where lithium is introduced into the cell in another way.Thus, lithium may be incorporated in the cathode material which isassembled into the cell. In this case, the cell, when assembled is inthe discharged state. The carbonaceous composition is lithiated byapplying an externally generated electrical potential to recharge thecell and draw lithium from the cathode material, through the electrolyteand into the carbonaceous material of the anode. This approachordinarily is most practical where the cathode material, in itslithiated form is stable in air and hence can be handled readily.Examples of such air-stable lithiated cathode materials includelithiated nickel oxide, lithiated cobalt oxides and lithiated mixedoxides of cobalt with nickel or tin. Among the suitable oxides areLiNiO₂ ; LiCoO₂ ; LiCo₀.92 Sn₀.08 O₂ ; and LiCo_(l-x) Ni_(x) O₂.According to a variant of this approach, the particulate carboncomposition can be lithiated before it is introduced to the cell.However, this approach suffers from the difficulties attendant uponhandling of lithiated carbon outside of the cell. Lithiated carbontypically tends to react strongly with air. Regardless of the source oflithium or the location of the lithiation, however, the temperatureduring lithiation desirably should be conducted at about 20° C. or belowexcept that when the carbonaceous composition includes a mono-phase,highly graphitized carbon lithiation should be conducted at an elevatedtemperature as noted above.

The cells can be made in a wide variety of physical configurations. Forexample, the spiral configuration illustrated and discussed above isentirely optional. Likewise, the current collectors employed in theillustrated cells may be omitted. Thus, cells without current collectorsmay incorporate substantially button-shaped electrodes made by acompaction process. A slurry prepared as above may be continuously mixedin an open reaction vessel while the solvent evaporates. The remainingdry material is broken up to form an essentially free flowing powderincorporating the filamentary material and particulate composition inintimate admixture. This powder may then be consolidated in anappropriate press in order to form electrodes of the desired shape.Powder consolidation processes of this type may be used to formbutton-shaped electrodes. Preferred button-shaped carbonaceouselectrodes formed by this process have a density between about 0.5 gm/ccand about 2.0 gm/cc; more preferable are electrodes having a densitybetween about 0.8 gm/cc and about 1.4 gm/cc.

The button-shaped electrodes may be disposed on opposite sides of aseparator and connected to opposite sides of a generally button-shapedcell housing. Inasmuch as the preferred cells according to the presentinvention do not rely upon stack pressure, cells can be made withoutseparators. For example, the anode and cathode may be disposed remotefrom one another within a cell container and mounted to respective,electrically isolated portions of the cell container so that the twoelectrodes do not touch one another.

The following non-limiting examples illustrate certain aspects of theinvention.

EXAMPLE I

Several test cells are made, each incorporating a button-typecarbonaceous anode, a lithium electrode and an electrolyte consisting of1 M LiAsF₆ in a solvent of equal volumes of propylene carbonate andethylene carbonate. These test cells do not incorporate a cathode activematerial but rather are arranged for transfer of lithium only betweenthe lithium metal electrode and the carbonaceous anode. The lithiumelectrode is spaced apart from the carbonaceous anode by a porous,non-conductive separator, so that electrical contact between the lithiumand carbon electrodes can only be made through an external circuit. Theexternal circuit is provided with appropriate devices for controllingcurrent during discharge of the cell (during spontaneous transfer oflithium to the carbon electrode) and for applying electrical potentialto recharge the cell (by transferring lithium from the carbonaceouselectrode to the lithium electrode). The external circuit also includesdevices for monitoring the voltage between the lithium and carbonelectrodes. The anode of each cell incorporates petroleum coke as aparticulate carbonaceous composition together with 2% EPDM polymerbinder on a copper expanded metal current collector. Varying amounts ofcarbon black are added to the anodes of different cells as indicated inTable I. The cells are cycled repeatedly and tested for capacity fadewith results as shown:

                  TABLE I                                                         ______________________________________                                                           STACK        AVERAGE                                                PERCENT   PRESSURE     % DECREASE                                             CARBON    (Pounds Per  IN CAPACITY                                   CELL     BLACK     Square Inch) PER CYCLE                                     ______________________________________                                        A        0         200          0.1                                           B        0         0            90                                            C        2         0            10                                            D        3         0            4                                             E        5         0            0.6                                           ______________________________________                                    

These results demonstrate the effect of the conductive, filamentarycarbon black in minimizing capacity fade.

EXAMPLE II

Test cells are made substantially in accordance with Example I, exceptthat the anodes of all of the cells of this Example incorporatesubstantially pure graphite, having a degree of graphitization equal toabout 1.00, a particle size of about 200 to about 400 mesh U.S. standardsieve size and a surface area of about 10 m² /gm with a 2% EPDM binder.All cells are operated under a stack pressure of 200 psi. The charge anddischarge cycles are controlled so that each full charge and dischargeoccurs over a period of 100 hours. The first discharge of each cell, andhence the first transfer of lithium into the carbonaceous electrodeoccurs at a temperature as specified in Table II.

                                      TABLE II                                    __________________________________________________________________________                  FIRST   SECOND  IRREVERS-                                           TEMPERATURE                                                                             DISCHARGE                                                                             DISCHARGE                                                                             IBLE                                                AT FIRST  CAPACITY                                                                              CAPACITY                                                                              CAPACITY                                        CELL                                                                              CYCLE °C.                                                                        Ah/g    Ah/g    Ah/g                                            __________________________________________________________________________    A   -10       .390    .013    .377                                            B   +21       .658    .290    .368                                            C   +55       .450    .322    .128                                            D   +70       .431    .330    .101                                            __________________________________________________________________________

The second charge/discharge cycle for all cells occurs at roomtemperature (approximately 21° C.). The capacity on the first dischargeincludes both reversible intercalation of lithium and the irreversiblereaction related to decomposition of the electrolyte solvent. Thecapacity on the second cycle includes only the reversible capacityrelated to intercalation and deintercalation of lithium. The differencebetween the capacities on the first and second cycles yields theirreversible capacity, a measure of the magnitude of the irreversibledecomposition reaction, as indicated in Table II.

EXAMPLE III

The procedure of Example II is repeated using two cells. One cell has acarbonaceous electrode incorporating as the particulate carbonaceouscomposition isotropic graphite of the type sold under the designationGSI-EC110 by a Graphite Sales Inc., of Chagrin Falls, Ohio, U.S.A.Another cell incorporates graphite of the type referred to as highlycrystalline pure graphite and sold under the designation KS-15 by theLonza Company of Fair Lawn, New Jersey, U.S.A. The initial lithiation isconducted at 21° C. During the initial lithiation, the voltage acrossthe cell with the crystalline graphite electrode remains substantiallystable for a considerable period at about 0.7 volts, indicating thatsubstantial exfoliation is occurring. The current consumed during thisperiod of voltage stability at about 0.7 volts corresponds to anirreversible capacity caused by exfoliation of about 0.19 Ah/g. The cellwith the isotropic graphite does not exhibit any substantial period ofvoltage stability at about 0.7 volts, and has an irreversible capacitycaused by exfoliation of less than 0.01 Ah/g.

EXAMPLE IV

A series of cells are made and tested substantially in accordance withExample III using monophase carbons having differing degrees ofgraphitization. The results are as indicated in Table III.

                  TABLE III                                                       ______________________________________                                                                IRREVERSIBLE                                                                  CAPACITY DUE                                                   DEGREE OF      TO EXFOLIATION                                        CELL     GRAPHITIZATION (Ah/g)                                                ______________________________________                                        A        .155           0                                                     B        .207           0                                                     C        .236           0                                                     D        .322           .016                                                  E        .437           .130                                                  F        .597           .180                                                  G        .604           .190                                                  H        .903           .185                                                  I        1              .185                                                  ______________________________________                                    

These results indicate that a carbon having a degree of graphitizationof about 0.4 or below is substantially resistant to exfoliation.

EXAMPLE V

An AA size test cell is fabricated generally in accordance with FIGS. 1and 2 using petroleum coke as the particulate carbonaceous compositionof the anode. The carbonaceous anode incorporates about 5% carbon blackof the type sold under the designation Super S, and having a surfacearea of about 40 m² /g. The cathode material is a manganese dioxidetreated in accordance with co-pending U.S. Patent Application No.07/217,668. The separator is a microporous polypropylene fabric, and theelectrolyte is 1M LiAsF₆ in mixed propylene carbonate and ethylenecarbonate. Although the cell is spiral wound under tension with alithium foil sheet, and hence has some stack pressure when initiallyassembled, this stack pressure substantially dissipates when the lithiumis transferred from the sheet to the carbon electrode. A control cell ismade the same way but without the carbon black in the anode. The cellsare repeatedly cycled at 20° C. between a discharged voltage of 1.0voltsand a charged voltage of 3.3 volts using 20 mA charge and dischargecurrents. The test cell has an initial capacity of about 0.27Ah. Afterthe first cycles, the capacity of the test cell stabilizes at about0.24Ah and remains at about 0.23 Ah even after 40 cycles. The controlcell, without carbon black, has an initial capacity of about 0.25 Ah,which decreases to about 0.02 Ah in only 3 cycles, at which pointcycling of the control cell is halted.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principals and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

We claim:
 1. A rechargeable battery comprising a first electrode and acounterelectrode each capable of reversibly incorporating an alkalimetal, an alkali metal incorporated in at least one of said electrodes,and an electrolyte comprising an organic solvent and a salt of saidalkali metal wherein said first electrode includes a particulatecomposition including carbon, substantially every particle of saidcomposition including a first carbonaceous phase and a secondcarbonaceous phase intimately admixed with said first carbonaceousphase, said first phase having a higher degree of graphitization thansaid second phase.
 2. A battery as claimed in claim 1 wherein saidcomposition has a mean degree of graphitization of at least about 0.40.3. A battery as claimed in claim 2 wherein said first phase comprisescarbon having a degree of graphitization greater than about 0.40 andsaid second phase comprises carbon having a degree of graphitizationless than about 0.40.
 4. A battery as claimed in claim 1 wherein insubstantially every particle of said composition said first phaseincludes a plurality of graphitic domains.
 5. A battery as claimed inclaim 1 wherein said composition comprises spherical graphite.
 6. Abattery as claimed in claim 1 wherein said composition comprisesisotropic graphite.
 7. A rechargeable battery comprising a firstelectrode, and a counterelectrode each capable of reversiblyincorporating an alkali metal, an alkali metal incorporated in at leastone of said electrodes and an electrolyte comprising an organic solventand a salt of said alkali metal wherein said first electrode comprises aparticulate composition including carbon, said composition having asurface area subsequent to intercalation with said alkali metal which issubstantially the same as surface area of said composition prior tointercalation with said alkali metal, and wherein at least a portion ofsaid composition is carbon having a degree of graphitization greaterthan about 0.40.
 8. A rechargeable battery as claimed in claim 9 whereinsaid first electrode includes a composition consisting essentially ofcarbon having a degree of graphitization greater than about 0.40 whichhas been intercalated with lithium at a temperature greater than about50° C.
 9. A battery as claimed in claim 7 wherein said composition is inparticulate form.
 10. A battery as claimed in claim 1 or claim 8 orclaim 9 or claim 7 wherein said alkali metal consists essentially oflithium.
 11. A battery as claimed in claim 1 or claim 8 or claim 7wherein said alkali metal is present in intercalated form in at leastone of said electrodes.
 12. A battery as claimed in claim 1 or claim 9or claim 7 wherein said particulate composition has a surface area priorto intercalation of said alkali metal less than about 10 m² /g.
 13. Abattery as claimed in claim 1 or claim 9 or claim 7 wherein said firstelectrode further includes carbon black interspersed with saidparticulate composition.
 14. A rechargeable battery as claimed in claim13 which is substantially free of pressure on said first electrode. 15.A rechargeable battery as claimed claim 13 wherein said carbon black hasa surface area of less than about 50 m² /gm.
 16. A rechargeable batterycomprising a first electrode and a counterelectrode each capable ofreversibly incorporating an alkali metal, an alkali metal incorporatedin at least one of said electrodes and an electrolyte including anorganic solvent and a salt of said alkali metal wherein said firstelectrode includes a particulate composition including carbon and carbonblack interspersed with said particulate carbon composition, and whereinsaid battery is essentially free of pressure on said first electrode.17. A battery as claimed in claim 16 wherein said alkali metal consistsessentially of lithium.
 18. A battery as claimed in claim 16 whereinsaid carbon black has a surface area of less than about 50 m² /gm.
 19. Abattery as claimed in claim 18 wherein said carbon black has a surfacearea of about 40 m² /gm.
 20. A battery as claimed in claim 16 whereinsaid first electrode includes between about 1 wt. % and about 12 wt. %of said carbon black.
 21. A battery as claimed in claim 20 wherein saidfirst electrode includes between about 4 wt. % and about 7 wt. % of saidcarbon black.
 22. A rechargeable battery comprising a first electrodeand a counterelectrode each capable of reversibly incorporating analkaline metal, an alkaline metal incorporated in at least one of saidelectrodes and an electrolyte comprising an organic solvent and a saltof said alkaline metal wherein said first electrode includes aparticulate composition including carbon, said particulate compositionhaving a surface area of less than about 8 m² /gm and a degree ofgraphitization less than about 0.40.
 23. A battery as claimed in claim22 wherein said alkali metal consists essentially of lithium.
 24. Abattery as claimed in claim 22 wherein said composition has a surfacearea of less than about 6 m² /gm.
 25. A battery as claimed in claim 22wherein said composition has a surface area of less than about 4 m² /gm.26. A battery as claimed in claim 22 wherein said composition has asurface area of less than about 2 m² /gm.
 27. A battery as claimed inclaim 22 wherein said composition has a surface area of at least about0.03 m² /g.
 28. A battery as claimed in claim 22 wherein saidcomposition consists essentially of coke.
 29. A battery as claimed inclaim 28 wherein said composition consists essentially of petroleumcoke.